There are areas surrounding a work machine such as a hydraulic excavator that cannot be visually checked with ease by an operator in the operator's cab. Namely, the operator cannot see the areas to the left, to the right and to the rear of the operator's cab. For this reason, it is desirable to install a monitoring device that monitors those areas for any obstacle or person that may be present in order to prevent accidents.
Work machine monitoring devices known in the related art include the following (patent literature 1). A proximity sensor that detects an object present within a monitoring area around a work machine and an imaging device that captures an image of a detection target area allocated to the proximity sensor are installed in the work machine in the related art. The monitoring device is capable of determining the position of an object present in the monitoring area based upon the length of time elapsing before an ultrasound wave output from the proximity sensor is reflected by the object. Upon detecting that there is an object in the monitoring area, the monitoring device displays information indicating the position of the object, together with the image captured by the imaging device as described above, at a display device.